The invention relates to ceramic wall-flow filters and methods of making them. In particular, the invention relates to particulate traps, such as diesel particulate traps.
As air quality standards become more stringent, considerable efforts have focused on minimizing the particulate matter emitted in diesel engine exhaust. A potential solution is a particulate trap inserted in the exhaust system of a diesel engine.
A honeycomb ceramic wall-flow through filter, such as described in U.S. Pat. No. 4,276,071, has become the preferred type of particulate trap. These honeycomb filters are made by extruding a paste comprised of water, binder and ceramic powders (e.g., clay, mullite, silica, silicon carbide and alumina) that form, for example, cordierite upon firing. Clays or water soluble binders are generally used to make the paste sufficiently plastic to form useable honeycombs. After the paste is extruded, the honeycomb is dried, debindered and sintered to form a honeycomb. The honeycomb is sintered, typically, to give sufficient strength to the thin channel walls to survive insertion of a ceramic paste to plug the channels, as described next.
Finally, to make the wall-flow particulate trap or filter, one half of the openings of one end of the sintered honeycomb are plugged with a paste comprised of a suitable powder, dispersion medium and binder. Then, on the other end, the channels not already plugged are plugged with the paste. Subsequently, the plugged honeycomb is sintered again to form the wall-flow particulate trap.
Unfortunately, this method suffers from a number of problems. For example, the liquid in the paste may be drawn into the porous walls of the fired honeycomb preferentially causing non-uniform drying shrinkage of the plug and ultimately cracks in the plug. A second problem is the necessity for multiple expensive steps (e.g., at least two high temperature firings) to manufacture the particulate trap. These multiple steps are typically needed because the walls of a green ceramic honeycomb are thin and fragile they tend to deform and/or break when inserting the paste, which is particularly true when using a large scale process. Another problem is the limited compositions that may be used for the plug, due to the expansion of the fired honeycomb during the sintering shrinkage of the plug.
Accordingly, it would be desirable to provide a method for making wall-flow traps, for example, that avoids one or more of the problems of the prior art, such as one of those described above.
The invention is a method of plugging channels in a ceramic honeycomb comprising:
(a) forming a mixture comprised of a dispersing liquid and ceramic powder,
(b) inserting the mixture at one end of the channel in an unplugged ceramic honeycomb, such that the mixture flows to the other end where the mixture collects and forms a plugged ceramic honeycomb and
(c) heating the plugged ceramic honeycomb to a temperature sufficient to sinter the plugged ceramic honeycomb to form a porous sintered plugged ceramic honeycomb.
The method is particularly useful, but not limited to, plugging of channels in honeycomb wall-flow filters. Surprisingly, these methods may be used to not only plug the channels, but also simultaneously provide a discriminating layer, for example, on the walls of the outlet channels of a ceramic honeycomb wall-flow filter. The method may also be used to provide other useful materials on or in the walls of the filter (e.g., a catalyst or nucleation agent) of a channel, while simultaneously forming the plugs.